The invention relates to a transport system for the shell assembly of vehicle bodies, the said transport system having
a) a rail system connecting at least two processing stations;
b) at least one carriage which is capable of travelling on the rail system by means of a driving arrangement and on which the workpiece can be fastened;
c) positioning means which bring about the exact spatial location of the workpiece at each processing station, namely
ca) in the vertical direction;
cb) in the direction transverse to the rail system;
cc) in the direction of the said rail system.
In the so-called xe2x80x9cshell assemblyxe2x80x9d of vehicle bodies, the various parts of the body are connected to one another by spot-welding operations at a number of processing stations with the aid of robots. The robots are programmed in such a way that they place their welds at precisely defined coordinates in space. The task of the transport system, which guides the vehicle body being produced through the various processing stations, is to guide the said vehicle body being produced (which is also called xe2x80x9cthe workpiecexe2x80x9d here) precisely to the correct point, within a fraction of a millimeter, so that the spot-welding operations performed by the robots lie at the correct points on the workpiece. Under these circumstances, the positioning of the workpiece must take place along three axes, namely in the direction of movement of the carriage, transversely to the direction of movement of the carriage and in the vertical direction. In addition to this difficult task, the transport system must be very rapid, the concept of xe2x80x9crapidityxe2x80x9d referring to the acceleration of the carriage out of, and deceleration of the said carriage into the processing station, to the speed between the processing stations, and also to the change-over times required in the particular processing station.
A transport system of the type initially mentioned is known from the trade point of view. In this case, the driving arrangement comprises a rotating driving shaft which extends between the rails of the rail system, parallel to the said rails, and which can be brought into connection, in a thread-like manner, with a driving nut on the carriage and in this way conveys the said carriage along the rail system. The positioning of the carriage transversely to the direction of movement takes place by highly precise guidance of the wheels of the carriage on the rails; and the vertical position of the components-carrier located on the carriage is taken care of by a separate lifting arrangement. Finally, the correct position of the components-carrier in the direction of movement of the carriage is guaranteed by arresting devices which are located in the particular processing stations and latch in at corresponding receptacles on the said carriage when the latter has reached the correct position in the direction of movement.
This known transport system is very costly: Above all, it is extremely difficult to guarantee lateral guidance of the wheels of the carriage on the rail system with the necessary precision over the entire length of the transport system. The separate lifting arrangement needs a drive of its own and expensive mechanical components which ensure that the components-carrier actually reaches the desired vertical position. Finally, the locking devices which determine the position of the carriage in the direction of movement have to be unlocked again by an active drive for the purposes of onward travel. However, each independent drive means additional susceptibility to breakdowns of the transport system as a whole. Since the transport system is the central constituent part of the entire shell assembly plant, stoppages of the said transport system can lead to extremely costly downtimes of the shell plant as a whole.
Skid conveyers are also employed as the transport system for the shell assembly of vehicle bodies, the skids being provided with arrangements which permit a combined lifting and horizontal movement of the components-carrier. The disadvantages of this known system correspond, as far as the necessity for separate driving arrangements for positioning purposes is concerned, to those of the transport system already described above. Moreover, the accelerations and speeds that can be achieved with skid conveyer systems are not particularly great.
Electric telpher lines are also employed as the transport system in the shell assembly of vehicle bodies. These electric telpher lines deposit the components at the various processing stations, the consequence of this being long change-over times. Similar disadvantages arise if so-called xe2x80x9cdriver-less transport systemsxe2x80x9d, that is to say computer-controlled, freely movable carriages are used as the transport system.
The object of the present invention is to further develop a transport system of the initially mentioned type in such a way that it is cheaper to manufacture and less susceptible to breakdowns.
This object is achieved, according to the invention, through the fact that
d) the positioning means are positioning means which work passively without a drive of their own, are activated solely by the movement of the carriage and are independent of the rail system, and which comprise:
da) positioning means which are disposed in a stationary manner at each treatment station;
db) positioning means on the carriage which cooperate with the stationary positioning means;
e) the nature of the guidance of the carriage on the rail system and/or the design of the carriage permits a certain movement of the workpiece transversely to the direction of the rail system and in the vertical direction when corresponding forces act on the said carriage.
According to the invention, therefore, precise guidance of the carriage, or of the components-carrier located thereon, in the direction transverse to the direction of movement throughout the path of movement between the processing stations, such as was the case in the prior art first mentioned above, is dispensed with. Instead, the location of the components-carrier or of the carriage in the transverse direction, as well as in the vertical direction and in the direction of movement, is brought about only in the particular processing station itself. The positioning means employed for this purpose are purely passive. That means that they are able to dispense with driving means of their own and fall back exclusively on a xe2x80x9ccamxe2x80x9d or xe2x80x9clinkxe2x80x9d action on the part of cooperating stationary positioning means and positioning means on the carriage. The forces which are needed for positioning the components-carrier are derived exclusively from the movement of the carriage as a result of this xe2x80x9ccamxe2x80x9d or xe2x80x9clinkxe2x80x9d action. In order to make the correct positioning of the components-carrier possible at all by means of the said passive positioning means, it is necessary for the components-carrier of the carriage to have a certain mobility transversely to the direction of the rail system and in the vertical direction of the said rail system. All in all, the transport system according to the invention constructed in this way is very inexpensive, since high-quality guidance of the carriage on the rail system is not necessary. The cost-effectiveness is also a consequence of the absence of separate drive systems for the positioning means, which at the same time distinctly reduces the susceptibility to breakdowns.
It is advantageous if the positioning means which bring about the correct position of the workpiece in the vertical direction and the positioning means which bring about the correct position of the workpiece transversely to the direction of the rail system comprise positioning ramps which have an oblique run-on face and a positioning face which adjoins the latter and extends parallel to the direction of movement. Under these circumstances, the positioning ramps may be located on the carriage; in that case, the oblique run-on face precedes the positioning face in the direction of movement. If, on the other hand, the positioning ramps are stationary, the run-on face points towards the direction of movement of the carriage. In both cases, the positioning face, in cooperation with the corresponding counter-positioning means, determines the correct position of the components-carrier, while the run-on face facilitates a displacement of the said components-carrier which is necessary when the latter runs onto the positioning ramp.
The positioning means which cooperate with the positioning ramps are expediently positioning rollers. Positioning ramps and positioning rollers are capable of cooperating in a substantially friction-free manner.
It is also of advantage if the positioning rollers which determine the vertical position of the workpiece are mounted so as to be displaceable in the horizontal direction transversely to the direction of movement of the carriage. That is to say, the said positioning rollers can then move with the carriage when the latter moves transversely to the direction of the rail system, as a result of which sliding of the said positioning rollers on the corresponding counter-positioning means which is subject to friction is avoided. The said displaceable positioning rollers are expediently pressed by springs into a central position transversely to the direction of movement, so that they therefore return again and again into a defined starting position after the last carriage has travelled away.
It is recommended, particularly in the case of fairly large carriages, that they have positioning means at different positions in the longitudinal direction, which positioning means cooperate, in each case, with different stationary positioning means which lie one behind the other in the direction of movement, the spatial disposition of the positioning means being such that the positioning means which are disposed furthest forward on the carriage in the direction of movement are able to interact only with the positioning means which lie furthest in advance in the direction of movement. For example, it is recommended, in the case of fairly long carriages, that positioning means on the said carriage be attached in the region of the front and rear ends of the latter. The front and rear positioning means on the carriage must cooperate, in the processing station, with stationary positioning means which lie further in advance in the direction of movement and with stationary positioning means which do not lie so far in advance in the direction of movement. So that the front positioning means on the carriage do not already become active on reaching the first stationary positioning means, their relative position is selected in such a way that they do not lie in the mutual path of movement. Only the stationary positioning means which lie further in advance in the direction of movement have a location in which they are able to cooperate with the front positioning means on the carriage. That applies in a corresponding manner to the positioning means which lie further back on the carriage and which, because of their spatial disposition, are able to cooperate exclusively with those stationary positioning means for which they are intended.
The positioning means which bring about the correct location of the workpiece in the direction of the rail system advantageously comprise a stationary stop and a stop on the carriage which interacts with the said stationary stop. In the direction of the rail system, therefore, the carriage is positioned at the correct point not by an arresting mechanism but merely by a stop. Without an additional locking device, the carriage is pressed against the stationary stop with the aid of a suitably configured driving arrangement.
In this connection, it is particularly advantageous if the stationary stop can be travelled over by the stop on the carriage during a forward movement of the carriage, but cannot be travelled over during a backward movement of the said carriage. In this case, the positioning of the carriage in the direction of the rail system comes about as follows:
The carriage is first of all moved beyond the stationary stop. The driving arrangement then reverses the direction of movement of the carriage, which is driven backwards against the stop which cannot be overcome in this direction. The onward movement of the carriage in the forward direction after completion of the work at the processing station in question can take place without the pulling-away of the stop. That is to say, the said stop also works exclusively xe2x80x9cpassivelyxe2x80x9d in the sense mentioned above.
In a particularly advantageous form of embodiment of the invention, an automatic current controller is provided which monitors the current consumption of the driving arrangement of the carriage and, when there is a change in the said current consumption caused by the stop position of the carriage being reached, emits a signal for shutting off the driving arrangement and/or a starting signal for the processing tool at the processing station.
Particularly large accelerations and decelerations and also high speeds between the processing stations are achieved if the driving arrangement comprises a large number of stationary pairs of pressing rollers along the rail system, which act, on both sides, on at least one lateral, horizontal flange strip on the carriage.
If this type of driving arrangement is used, a refinement of the invention is recommended in which the pairs of pressing rollers have a certain mobility in the direction of movement of the carriage and are pressed, against the direction of movement, into a normal position by a spring arrangement, and that a detection arrangement be provided which picks up a deflection of the corresponding pair of pressing rollers caused by the stop position of the carriage being reached, and thereupon emits a signal for shutting off the driving arrangement and/or a starting signal for the processing tool at the processing station.